Cranberries, by way of example, are a major commercial crop in the United States. Grown traditionally on a flat parcel of land the vines will cover the ground forming so called cranberry beds. The small, red football-like shaped fruit appears on short upright branches several inches above the ground and is harvested in the fall before freezing. Various mechanisms such as freezing, insect damage, physical damage due to harvest, etc. may weaken the fruit and cause it to become susceptible to bacteria, decay, and early spoilage. When selling fresh cranberries it is desirable to present only unspoiled berries. Spoiled berries not only deter consumers but also cause sanitary issues in shipping and display because sticky juices from decaying fruit will leak onto containers, shelves and equipment. Softness and reduced structural integrity of the fruit is indicative of spoilage.
Cranberries intended for sale as fresh fruit are handled carefully after harvesting. They are taken to receiving stations where they are cleaned and stored prior to packaging. Just prior to packaging the fruit is sorted to remove spoiled berries and then placed in distributable containers. Various degrees of sorting are also performed to remove spoiled berries in what is considered upgraded fruit, sold as baking ingredients for example. Cranberries intended for juicing are typically least sorted.
For small operations sorting fresh cranberries can be an important way to add value to a limited crop volume as the price per pound is higher than for juice berries. Sorting can be done manually but is often augmented by a machine or some type of automation. For larger operations there is generally a greater reliance on automation.
While manual sorting can be important as a final check to automated sorting, reliance on manual sorting to remove the bulk of spoiled fruit is slow and unreliable. Manual sorting usually depends on a multitude of variables associated with an individual human sorter. In agriculture, an individual human sorter may employ multi-sensory functions, such as vision, touch, smell, hearing, and taste to examine the quality of a product. However, the manual sorting can vary widely from individual to individual, which introduces unwelcome fluctuations in the end product's quality. The length of the sorting time may also play a role. Manual sorter's ability could be affected by time of day or night, feeling tired or rested, quality of vision and touch. Physical and emotional states, judgment, and human reliability can have a detrimental impact on the end product. Overall, manual sorting of large quantities of small objects tends to be expensive and inefficient. Also, finding and employing steady seasonal workers can be a significant hurdle. As the product's quality and quantity are affected by and remain vulnerable to manual labor's multi-factor fluctuations, farmers, food, and drink producers seek other solutions. To limit dependency on manual workers, to preserve qualitative consistency, to increase processing rate, quality and quantity of throughput, automated sorters, also known as separators, are increasingly utilized.
Various sorting techniques have been used to assist in the automated sorting of cranberries for removal of spoiled berries. The techniques could be classified loosely in two categories as 1) using mechanical probing mechanisms and 2) using optical probing mechanisms. Mechanical sorting mechanisms disclosed to date employ a variety of techniques to remove rotten, soft, or undesirable berries. The techniques include: a) “Bailey Mills” which sort by bouncing the fruit over a small hurdle and measure the fruit's elasticity without accounting for rigidity, b) “Puff Ball” machines which attempt to push fruit through a small opening and measure the fruits rigidity without measuring elasticity. Such rough handling of fruit compromises the quality of the end product even for initially high quality berries which pass the sorter. Thus this method is undesirable to use for fresh packaging where even good fruit becomes damaged due to the sorting process' mechanism. c) “Tactile Sorting” machines which employ a force transducer to create an electrical signal and then process the signal electronically and decide whether the fruit should be removed.
References exist for tactile sorting techniques using piezo transducers under various configurations, which bring a mobile sensor into contact with the object under test but none disclose a method for delivering the objects to a stationary sensor. Also, none disclose a method of processing signals to account for an objects elasticity and rigidity simultaneously using either analog or digital electronics.
Piezoelectric or piezo sensors are electromechanical systems that react to compression. The word “piezo” is derived from the Greek “piezein,” which means to squeeze or press. The piezoelectric effect finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultra fine focusing of optical assemblies. The piezoelectric effect has further found a sorting application.
Piezoelectricity is the ability of some crystals and ceramics to generate an electric potential in response to applied mechanical stress. Piezoelectric sensors convert pressure, acceleration, strain, or force to an electrical signal. When pressure or touch is exerted upon a piezo sensor, a separation of electric charge across the crystal lattice occurs and can be measured as voltage potential. Piezo sensors show almost zero deflection with a high natural frequency and an excellent linearity over a wide amplitude range. Piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. The high modulus of elasticity of many piezoelectric materials is comparable to that of many metals. The piezoelectric effect is reversible, meaning, materials exhibiting a direct piezoelectric effect, that is, the production of electricity when stress is applied; also exhibit a converse piezoelectric effect, the production of stress or strain when an electric field is applied.
Although primarily described in an agricultural context, and in particular the embodiment of cranberry sorting system, such a sorting system could apply not only to a variety of other fruits and vegetables but also to any other field where sorting of small objects, based on a objects physical structure, namely elasticity and rigidity is desired.